Doped dlc for tribological applications

ABSTRACT

The present invention relates to a non-hydrogenated transition metal-doped diamond-like carbon (DLC), wherein the non-hydrogenated DLC comprises at least one transition metal selected from groups 4 d,  5 d  and 6 d  of the periodic table of elements. A part of the at least one transition metal is present in the form of carbide of the at least one transition metal in the non-hydrogenated DLC as a matrix, and another part is present as metal droplets. The non-hydrogenated transition metal-doped DLC has an indentation hardness of ≥35 GPa, preferably of ≥40 GPa. Due to the presence of metal droplets, the doped DLC is highly effective in improving wear resistance and/or reducing friction of a surface when a coating of the material is applied on the surface. Further, the present invention provides a cathodic arc discharge deposition method for depositing a coating of the non-hydrogenated transition metal-doped DLC according to the present invention.

FIELD OF THE INVENTION

The present invention relates to non-hydrogenated transition metal-doped diamond-like carbon (DLC), a use thereof for reducing friction of a surface, and a layer system comprising a coating thereof provided on a substrate. Moreover, the present invention relates to a method of depositing a coating of the non-hydrogenated transition metal-doped DLC according to the present invention, which method is a cathodic arc discharge deposition method.

BACKGROUND OF THE INVENTION

Diamond-like carbon (DLC) is a metastable form of amorphous carbon. DLC films have found widespread applications in science and technology. J. Robertson, in Materials Science and Engineering R 37, 2002, 129-281 provides a comprehensive overview of the material and its applications. As reported by Robertson, a distinction needs to be made between hydrogenated and non-hydrogenated forms of DLC. Another criterion for the classification of DLCs is the fraction of sp³ bonding. If for non-hydrogenated material the fraction of sp³ bonding reaches a high degree, the material is generally denoted as tetrahedral amorphous carbon, abbreviated as ta-C.

S. Xu et al. in Philosophical Magazine Part B, 76 :3, 351-361 report the deposition of ta-C films by the filtered cathodic vacuum arc technique on silicon at room temperature. High sp³ bond fractions (about 80% or higher) were obtained. Compressive stresses in the range of 7.5 to 12 GPa and hardnesses of 20 to 55 GPa were found. The maximum hardness was found to coincide with the highest sp³ fraction as determined by electron-energy-loss spectroscopy (EELS).

Doped carbon coatings have also been developed. A. Abou Gharam et al. in Surface and Coatings Technology 206 (2011), 1905-1912 study the high temperature tribological behavior of W-DLC against aluminum. The W-DLC coatings were deposited using a physical vapor deposition (PVD) system. The addition of W to hydrogenated DLC resulted in the reduction of the friction coefficient in the temperature range of 400° C. to 500° C. The problem is, that doping of hydrogenated DLC, e.g. with W, takes place at an atomic level by sputtering. Non-hydrogenated DLC (ta-C) has a better wettability against a number of lubricants and a better temperature stability than hydrogenated DLC.

Z. Wang et al., International Journal of Hydrogen Energy 42 (2016), 5783-5792 deposited W doped carbon on stainless steel substrates. They used a close-field unbalanced magnetron sputtering ion plating (CFUBMSIP) system with a bias voltage of −60 V. The CFUBMSIP system was equipped with one tungsten (W) target, two graphite targets, and, for the deposition of a thin Cr seed layer and a thin intermediate MC_(x) layer (M being Cr and W), with one chromium (Cr) target.

K. Hou et al. deposited niobium (Nb)-doped amorphous carbon (a-C) films on stainless steel substrates. Like Z. Wang et al. in the above-mentioned scientific article, they used a CFUBMSIP system. The bias voltage was −100 V. The CFUBMSIP system was equipped with one Nb target, two graphite targets, and moreover one titanium target. Niobium carbide is claimed to be embedded in the a-C matrix, and the presence of pure niobium is reported. Simulated first principle calculations assuming one Nb atom in the supercells gave simulation results suggesting a sp³ fraction of about 58%. From the fitting of XPS data, a sp³ fraction of up to 54% was derived. The maximum sp³ fraction of 54% was however obtained for a film, in which the cathode current of the Nb cathode was kept at zero, so no Nb sputtered from the target. The Raman results for the Nb-doped a-C films however suggest lower sp³ fractions. In particular, the I_(D)/I_(G) ratios of about 2.5 suggest this when considering the teaching of A. C. Ferrari in Diamond and Related Materials 11 (2002), 1053-1061. No hardness data are reported in the article by K. Hou et al.

D. Zhang et al. (Carbon 145 (2019), 333-344) have doped amorphous carbon films with silver (Ag) or co-doped the films with Ag and chromium using a CFUBMSIP system. They have seen that the higher percentages of dopant resulted in a lower hardness and graphitization. They report a simulated hardness showing rather high hardness values. For pure C, their simulation shows a simulated hardness of 56 GPa. However, their measurements showed a compressive stress for all doped C-coatings between 2.40 and 3.37 GPa.

M. Andersson et al. (Vacuum 86 (2012), 1408-1416) deposited and characterized magnetron sputtered amorphous Cr—C films. They used non-reactive DC magnetron sputtering from elemental targets. The films were found to be X-ray amorphous with no presence of crystallites. They report a hardness of 6.9 GPa for a Cr dopant level of about 15 at. % (i.e. atomic %) increasing to 10.6 GPa for about 75 at. % Cr.

Y. Lin and S. Zhang in J. Nanosci. Nanotechnol. 16 (2016), 12720-12725) studied the effect of Cr addition on the properties of graphite-like carbon (GLC) films. The films are deposited by unbalanced magnetron sputtering. They report a hardness of 10.4 GPa for pure C and of up to 17.4 GPa for GLC films doped with Cr. Dopant levels are not expressly mentioned but for the Cr target power increasing from 0.1 kW to 0.3 kW and for carbon increasing from 0 to 5 kW, fairly low dopant levels can be expected.

A. Amanov et al. (Tribology International 62 (2013), 49-57) deposited Cr-doped and non-doped DLC films using unbalanced magnetron sputtering (UBMS). They report a hardness of 22.47 GPa for Cr-doped DLC, and of 10.75 GPa for non-doped DLC.

A. Ya. Kolpakov et al. (Nanotechnologies in Russia, 5 (2010), 160-164 employ a pulse vacuum arc method to deposit ta-C coatings doped with nitrogen, tungsten or aluminum. The level of dopant, such as tungsten, is not mentioned, nor the amount of W in the composite graphite-based cathode. The repetition frequency of pulses in the pulse vacuum arc method was 2.5 Hz. The doped carbon coatings are described as amorphous in structure and without crystalline impurities. For the W-doped films, microhardnesses HV of up to below 20 GPa were found. There is no evidence for pure W droplets found. The usage of arc discharge to create ta-C has also been described by R. H. Horsfall, Proc. Soc. Vacuum Coaters (1998), 60-85 using a DC arc discharge. Pulsed discharges to create ta-C have also been described V. N. Inkin et al., Diamond and Related Materials 13 (2004), 1474-1479.

Summarizing, with unbalanced magnetron sputtering excluding filters to filter the uncharged particles, a hardness for pure C is rather low, so far reported in the range of 20 GPa, whereas adding a substantial level of dopant, typically in the range of 10 at. % or higher shows an increase of hardness, caused by the higher percentages of Carbides going up to a hardness of about 22.5 GPa such as in the above-mentioned scientific article by A. Amanov et al.

As mentioned above, DLC films have found various applications in Science and Technology. Amorphous carbon films were also studied by some researches as protective coatings for bipolar plates (BBPs) in polymer electrolyte membrane (PEM) or proton exchange membrane (PEM) fuel cells (PEMFCs). BPPs have vital functions in PEMFCs and PEMFC stacks. For instance, they separate individual cells in a PEMFC stack, distribute the fuel gases and separate them, act as current collectors, facilitate heat and water removal, provide mechanical support for other components, and act as backbone of the FC stack.

For instance, Z. Wang et al., in the above-mentioned scientific article, deposited W-doped carbon by means of CFUBMSIP on austenitic stainless steel substrates and studied the interface contact resistance (ICR) and the corrosion resistance. They observed that a dopant level of W in between 2.54 at. % and 24.41 at. % increased the corrosion resistance. They obtained the best results for 2.54 and 8.56 at. %. They ascribe the corrosion protection to the formation of a tungsten oxide layer.

D. Zhang et al. in the above-cited scientific paper found that amorphous carbon (a-C) films doped with Ag and Cr simultaneously achieve low ICR. Moreover, their tests showed a lifetime improvement with less corrosion and less out-diffusion when used as coatings on metallic BPPs with dopant levels of 4.89 at. % Ag and 12.37 at. % Cr.

P. Yi et al. in International Journal of Hydrogen Energy 44 (2019), 6813-6843 provide a review of carbon-based coatings for metallic BPPs used in PEMFCs. Transition metal carbide (TMC) coatings are also mentioned in this review article.

DLC coatings have also found application in the industry to reduce friction and improve wear resistance. Generally hydrogenated diamond like carbon (a-C:H) has been applied for this purpose. Limitations of the usage of hydrogenated diamond like carbon are, the hardness, which is in the range of 20-35 GPa and the application temperature, which is limited to about 300° C. A further limitation is that the low friction depends on the presence of water. To improve the temperature stability, 4d, 5d or 6d transition metals elements like W are added to hydrogenated DLC. An additional effect of adding easily oxidizing transition elements like W, Ta, V to hydrogenated DLC is that the metal reacts with oxygen and adds lubricity at higher temperature. In the above-cited scientific article by A. Abou Gharam et al., it is shown that the addition of W to hydrogenated DLC results in reduction of the friction coefficient in the temperature range 400-500° C. The problem is, that adding dopants like W in hydrogenated DLC only takes place at an atomic level, by sputtering.

Physical and Chemical Vapor Deposition coatings (PVD and CVD including Plasma-Assisted Chemical Vapor Deposition, PACVD) and derivatives are used to enhance the performance of base materials. Performance improvements can be directed to improvement of for example the wear resistance, or reduction of the friction. To tailor the properties of the coatings, one can dope the coatings with additional elements, can alter the composition, texture, internal stress level. Generally doped coatings have been widely applied in the last 20 years. A lot of experiments have been done on the performance of transition metal doped hydrogenated DLCs (a-C:H:Me), for an overview see S. Yazawa et al., Lubricants 2 (2014), 90-112. A beneficiary effect of the addition of a transition metals is, that generally elements like W and Mo form sulfides, with good lubrication properties. Both for elements in contact with fuel and in contact with lubricants this may play a role, as these may contain sulfur. Furthermore, W has shown to have a beneficial effect on the friction coefficient for contacts lubricated with molybdenum dithiocarbamate (MODTC). A reference for the beneficial effect of tungsten built in in a DLC layer against different lubricants is given in B. Vengudusamy et al., Tribology International 54 (2012), 68-76. In WO 2014/000994 A1, a method is described to bring in W into a hydrogenated diamond like carbon coating by usage of a metallo-organic precursor. The main reason to apply this was to increase the deposition rate in the process.

In all above described methods, the transition metal elements are in the plasma phase at atomic level, the atoms will arrive at the surface mainly as single atoms. As there is in the above described experiments a parallel flow of Carbon, the transition metal will be built atomically into the a-C:H coating, whereby the transition metal is bonded to carbon, forming a carbide. A detailed description of e.g. Ti built into hydrogenated DLC (a-C:H) is given in W. J. Meng et al., J. of Appl. Phys. 88,(2000), 2415-2422.

In view of the above, there was a demand for tribological coatings, i.e. coatings reducing friction and improving wear resistance, which have improved temperature stability and better wettability against a number of lubricants.

In summary, there is a large interest in industry to provide a coating material, which has outstanding properties as a coating of BPPs in fuel cells and which moreover has excellent tribological properties.

SUMMARY OF THE INVENTION Non-Hydrogenated Transition Metal-Doped DLC

The present application satisfies these needs by providing a non-hydrogenated transition metal-doped diamond-like carbon (DLC) as defined in claim 1. Accordingly, the non-hydrogenated DLC comprises at least one transition metal selected from groups 4d, 5d and 6d of the periodic table of elements and a part of the at least one transition metal is present in the form of carbide of the at least one transition metal in the non-hydrogenated DLC as a matrix. The non-hydrogenated transition metal-doped DLC is characterized in that it has a hardness of 35 GPa, preferably of 40 GPa, wherein the hardness is measured on a film of the non-hydrogenated transition metal-doped DLC deposited on a polished substrate with an indentation depth less than 10% of the thickness of the film. Moreover, the non-hydrogenated transition metal-doped DLC is characterized in that another part of the at least one transition metal is present in the form of the metal as droplets of the transition metal.

In the present application, the non-hydrogenated transition metal-doped DLC according to the present invention as defined in claim 1 will occasionally be referred to for brevity as “doped DLC according to the present invention”. The transition metal selected from groups 4d, 5d and 6d present in the doped DLC according to the present invention is sometimes referred to herein for simplicity as “the transition metal” or abbreviated as “TM”.

The preamble of claim 1 is formulated in view of the scientific article by Z. Wang et al., according to which amorphous WC was observed, and the scientific article by K. Hou et al., according to which NbC phases were observed. However, the W-doped carbon films of Z. Wang et al. and the Nb-doped carbon films of K. Hou et al. have a hardness lower than 35 GPa.

When the hardness is below 35 GPa, the coatings are too soft.

For instance, the hardness of the doped DLC according to the present invention can be in the range of 40 to 60 GPa. The hardness of the doped DLC according to the present invention is more preferably 45 GPa.

The hardness of the doped DLC according to the present invention is indicated in this application as GPa. The hardness is measured by nano-indentation of a Vickers pyramid indenter on a film of the non-hydrogenated transition metal-doped DLC deposited on a flat polished hardened substrate with an indentation depth less than 10% of the film thickness. The flat polished hardened substrate has a surface roughness Ra of 0.01 μm and Rz of 0.25 μm. The hardness of the flat polished hardened substrate used was 83.6 HRa (Rockwell hardness A, HRA), 62,1 HR_(c) (Rockwell hardness C, HRC) and 747 HV10 (Vickers hardness at a load of 10 kgf). Details of measuring the hardness can be found in the section “Detailed description of preferred embodiments” of this application.

The doped DLC according to the present invention has a much higher hardness than the BPP coatings known from the literature, such as from Z. Wang et al. and K. Hou et al., which is caused by a higher fraction of sp³ bonds, which is typically above 60%. In non-hydrogenated DLCs, the hardness correlates with the sp³ fraction, i.e. the fraction of carbon atoms present in the sp³ bonded state in terms of the sum of carbon atoms in the material in the sp, sp² and sp³ hybridization state, i.e. sp, sp₂ and sp³-bonded carbon. The doped DLC according to the present invention typically has a sp³ fraction of ≥60%, preferably ≥70%, more preferably ≥80% and most preferably ≥85%.

Owing to its high sp³ fraction, the doped DLC according to the present invention can be denoted as ta-C, i.e. tetrahedral amorphous carbon.

Owing to its high hardness, and because it is non-hydrogenated DLC, the doped DLC according to the present invention is superior as a coating reducing friction and/or improving wear resistance of a surface, onto which it is applied.

Raman spectroscopy can be used to assess the sp³ fraction of carbon atoms and the presence of carbides of the transition metal(s), namely, the presence of TM-C bonds in the doped DLC according to the present invention. For instance, when the TM is W, the presence of TM-C bonds can be detected in Raman spectra by a peak in the range of 80-150cm⁻¹ for an excitation wavelength of 532 nm. Regarding the sp³ fraction, from the position of the G peak in Raman spectroscopy for the excitation wavelength used, as well as the ratio of the intensity of the D peak (I_(D)) and the intensity of the G peak (I_(G)), i.e. I_(D)/I_(G), the sp³ fraction can be estimated. This is described by A. C. Ferrari in his above-mentioned scientific article and illustrated in FIG. 2 of that article. The Raman excitation wavelength used here was 532 nm. For samples of the doped DLC according to the present invention, the position of the G peak was typically at 1.605 cm⁻¹, which for that excitation wavelength points to a sp³ fraction of carbon atoms over 60%.

In the present invention, the composition analysis of the doped DLC according to the present invention and the coatings of the material is preferably done by Electron Probe Micro Analysis (EPMA). Especially, the content of the at least one transition metal in the doped DLC according to the present invention can be determined by EPMA.

When the doped DLC according to the present invention is specified as “non-hydrogenated”, this means that no hydrogen is deliberately added during the deposition, in particular no significant amounts of hydrogen are added during the deposition of the material. However, a bit of hydrogen coming from water vapor in the system may be present and be incorporated in the doped DLC according to the present invention. Therefore, due to such sources of hydrogen, the “non-hydrogenated” doped DLC according to the present invention may have a hydrogen content of <1 at. %, in particular coming from water vapor in the system, for instance when the material is deposited with high productivity, and short pumping and short heating times of the deposition chamber.

For the same reasons as described above for hydrogen, small amounts of oxygen coming from water vapor in the system and trace air, and small amounts of argon coming from trace air and the inert gas atmosphere in the system may be present and be incorporated in the doped DLC according to the present invention. Therefore, due to such sources of oxygen and argon, the non-hydrogenated doped DLC according to the present invention may have an oxygen content of <1 at. % and an argon content of <1 at. %, in particular coming from water vapor, trace air and the inert gas atmosphere in the system, for instance when the material is deposited with high productivity, and short pumping and short heating times of the deposition chamber. Preferably, the oxygen content is <0.5 at. % and the argon content is <0.5 at. %. Most preferably, the oxygen content is <0.1 at. % and the argon content is <0.1 at. %.

The doped DLC according to the present invention is doped with at least one transition metal. The transition metal is selected from the groups 4d, 5d and 6d of the periodic table of elements. Thus, the at least one transition metal is selected from the group consisting of titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo) and tungsten (W). All of the above-mentioned transition metals for use in the invention can form carbides. Therefore, they can all be present in the form of carbide in the non-hydrogenated DLC as a matrix as required in the present invention.

The content of the at least one transition metal is not specifically limited, but is according to a preferred embodiment in the range of 0.1 to 5 at. % in terms of the non-hydrogenated transition metal-doped DLC. According to preferred embodiments, the content of the at least one transition metal in the doped DLC according to the present invention is 0.2 to 2.5 at. %, more preferably 0.3 to 2.0 at. % and most preferably 0.5 to 1.5 at or it is 1 to 5 at. %, more preferably 2 to 4 at. %.

When the doped DLC according to the present invention is for use as a tribological coating, i.e. for reducing friction and/or improving wear resistance of a surface, in particular for reducing friction of the surface, the content is favorably in the range of 1 to 5 at. %, more preferably 2 to 4 at. %. This is because it was found that for such higher dopant levels of the at least one transition metal, the proportion of metal droplets can be higher in the doped DLC according to the present invention in relation to the transition metal present as carbide, with the metal droplets having a lubricating effect.

The non-hydrogenated transition metal-doped DLC according to the present invention is preferably a homogenous material. This is because in the cathodic arc discharge deposition method of the invention, which can yield the doped DLC according to the present invention, homogenous coatings can be deposited. This is due to the fact that a carbon target doped with the at least one transition metal is used in the present invention as a target in the cathodic arc discharge. The literature examples using CFUBMSIP (such as Z. Wang and K. Hou in their above-mentioned scientific articles) have used cathodes with pure C and a pure metal, where the cathodes are either opposite (for 2 cathode systems, or at 90 degree (for 4 cathode systems) in the deposition chamber. Due to the rotation of the substrate table the concentration of the transition metal will in these cases display a layer-wise modulation of the transition metal content in a size order of 2 to 30 nm, depending on the rotation speed, deposition rate and equipment lay-out. This is because when for instance a TM target and two graphite targets are used in the CFUBMSIP method (such as employed by Z. Wang and K. Hou in their above-mentioned scientific articles), the TM content of the coating will be higher when facing the TM target during the rotation, and it will be a bit lower when facing the graphite target. In coatings of the doped DLC according to the present invention, the above nano-scale layer-wise modulation of the transition metal content in the thickness direction of the coating, in particular in size order of 2 to 30 nm can be avoided. That means, the at least one transition metal can be distributed uniformly (also when present as carbide or as a metallic droplet) throughout the coating, in particular in the thickness direction of the coating, in the doped DLC according to the present invention.

In the present invention, the at least one transition metal in the form of carbide is present in, i.e. distributed, preferably uniformly, within the non-hydrogenated DLC as a matrix. The size of the TM carbide phases or domains is not specifically limited. The carbides may be present as atomically distributed W—C units on the lower end, and as islands, in particular those having a size in the nm range, i.e. “nano-sized islands”, on the upper end. As used herein, the nanometer range, i.e. the nano-size is defined to include sizes from 0.1 nm to 100 nm. According to a preferred embodiment, the TM carbide is present atomically distributed up to nano-sized islands, more preferably up to nano-sized islands having a size of at most 2 nm. Still more preferably, the carbide of the at least one transition metal is present as nano-sized islands of the size of about 0.5 nm to 2 nm.

This proved beneficial for fulfilling the demanding requirements as a BPP coating, in particular for ensuring that there is covalently bound transition metal in the doped DLC according to the present invention, resulting in the desirable low ICR. Normally, very hard ta-C coatings have a high ICR. In the present invention, the size of the of the islands of the transition metal carbide can be determined by Transmission Electron Microscopy (TEM), in particular by bright field TEM (BFTEM) and high angle annular dark field with spot for scanning TEM (HAADF-STEM).

According to the present invention, another part, preferably the other part, of the at least one transition metal is present in the form of metal droplets. The droplets preferably have a diameter of less than 1 μm, preferably of 0.1 to 100 nm, preferably of 0.5 to 40 nm. Such small TM droplets that are distributed in the matrix of the doped DLC according to the present invention, and as such embedded in the coating of the material can form a particularly effective source of free transition metal. Also, the droplets of such sizes proved most effective to reduce friction of a surface when a coating of the doped DLC according to the present invention additionally comprising such droplets is applied on the surface.

To discriminate between the transition metal present in the form of carbide and the one present in the form of metal as droplets in the non-hydrogenated DLC as a matrix, Transmission Electron Microscopy (TEM) studies with bright field TEM (BFTEM) and with high angle annular dark field with spot for scanning TEM (HAADF-STEM) were carried out in the present invention. The combination of BFTEM and HAADF-STEM in conjunction with the Raman spectra allows the discrimination between the two forms of the transition metal present in the non-hydrogenated DLC matrix in the material according to the present invention. Moreover, the size of metal droplets could be determined by TEM, in particular by BFTEM and HAADF-STEM in the present invention.

Moreover, it will be shown for concrete embodiments below that W having a melting point of above 3,400° C. can form metallic droplets in the cathodic arc discharge deposition method according to the present invention that are present in the non-hydrogenated DLC matrix according to a preferred embodiment of the present invention. Thus, this will equally be possible for the other above-mentioned transition metals for use in the present invention having lower melting points.

According to a preferred embodiment, the part of the at least one transition metal present in the form of carbide in the doped DLC according to the present invention can be present in an amount of 60 at. % or less, in terms of the overall content of transition metal in the material, and is more preferably 50 at. % or less, still more preferably 40 at. % or less, further preferably 30 at. % or less, and most preferably 20 at. % or less. As found by the inventors, there is hardly any “free” transition metal (or segregated transition metal) in the doped DLC according to the present invention in the sense that it is neither in the form of carbide (such as in the form of carbide islands) nor in the form of metal droplets. That is, according to a preferred embodiment of the doped DLC according to the present invention, a total of 85 at. %, preferably 90 at. %, more preferably 95 at. % of the at least one transition metal, is present in the matrix of the non-hydrogenated DLC in the form of carbide (preferably as islands of the carbide) and/or in the form of metal droplets. Thus, the remainder of the at least one transition metal, i.e. the difference of the above-mentioned percentages of the amounts of transition metal present in the form of carbide to 100%, was found to be preferably present in the form of metal droplets of the transition metal. Therefore, in a preferred embodiment of the invention, the above-mentioned percentages of the transition metal in the form of carbide and the percentages of the transition metal in the of metal droplets will sum up to 100% of the transition metal present in the doped DLC according to the present invention.

Layer Systems

The doped DLC according to the present invention can form a layer system comprising at least one layer of it provided on a substrate. The thickness of the coating of the doped DLC according to the present invention and of the one or more layers of the doped DLC according to the present invention present in the layer system can be measured by SEM in the present invention.

The substrate may be cleaned by ion bombardment, in order to remove native oxides from the surface of the substrate. Ion bombardment of the substrate prior to deposition of a further layer on top of the substrate promotes adhesion of the further layer on top of the substrate. For example, the substrate can be cleaned by Ar etching, i.e. argon ions bombardment. In the layer system, an adhesion layer can be provided directly on the substrate, on top of which the at least one layer of the doped DLC according to the present invention can be formed. The adhesion layer can for instance be a layer of metallic Cr or metallic Ti. The layer system can also comprise a multilayer, with the multilayer comprising at least one layer of the doped DLC according to the present invention. Preferably, it is a multilayer of at least one layer of the non-hydrogenated transition metal-doped DLC, in which the content of the at least one transition metal is X at. % in terms of the layer, and at least one layer of the non-hydrogenated transition metal-doped DLC, in which the content of the at least one transition metal is more than 0 to 0.8 times X at. % in terms of the layer and/or at least one layer of ta-C. According to a preferred embodiment, there are thus at least two layers in the multilayer, one of which has a higher transition metal content, and one of which has a lower transition metal content (or no transition metal at all, which is the case for a layer of ta-C). There may be more than two alternating layers of high transition metal layer and low/no transition metal layer in the multilayer. The layer system can also comprise a transition layer between the adhesion layer and the single layer of the doped DLC according to the present or the multilayer of or comprising the doped DLC according to the present invention as described above, by a ramp down of the adhesion metal layer and ramp up of the doped DLC according to the present invention or the ta-C layer.

When the layer system according to the present invention comprises a single layer of the doped DLC according to the present invention, the thickness of the layer is preferably in the range of 50 nm to 3 μm, preferably in the range of 80 nm to 1 μm. This also applies to each of the layers of the doped DLC according to the present invention when present as or in a multilayer. The multilayer preferably has a thickness in the range of 0.1 to 30 μm, more preferably of 0.2 μm to 10 μm. This excludes the thickness of the optional adhesion layer and the optional transition layer present on the substrate and of course the thickness of the substrate as such.

Tribological Applications

The doped DLC according to the present invention turned out to have a better wettability against a number of lubricants and a higher temperature stability than hydrogenated DLC as described for instance in the above-mentioned scientific article by A. Abou Gharam et al. when used as a tribological coating. Non-hydrogenated DLC has an additional advantage that the wettability by a number of lubricants is superior over hydrogenated DLC, see M. Kano, Tribology International 39 (2006), 1682-1685. The doped DLC according to the present invention has a low friction stepping in already at 300° C., and it moreover has an improved wear resistance. Therefore, the present invention also pertains to the use of the doped DLC according to the present invention for reducing friction and/or improving wear resistance of a surface by coating the surface with doped DLC.

The effect of reducing friction is particularly pronounced because part of the at least one transition metal is present in the doped DLC according to the present invention in metallic form as droplets. Especially for higher contents of the at least one transition metal, such as in the range of 1 to 5 at. % in terms of the doped DLC according to the present invention, and for high proportions of the part of the at least one transition metal present in the form of metallic droplets in comparison to the part being present in the form of carbide, the metallic droplets will reduce the friction further. This is because the metallic transition metal droplets are embedded within the matrix of the material and can form a source of free transition metal having a lubricating effect.

As will be appreciated, the layer system as claimed, in particular the system with a single layer of the DLC according to the present invention of a thickness in the range of 0.5 to 3 μm with an optional adhesion layer between the substrate and the DLC layer according to the present invention proved highly useful for improving wear resistance and/or reducing friction of a surface of the substrate.

Deposition Method

Non-hydrogenated DLC having a hardness as high as 35 GPa, preferably of 40 GPa, as claimed in claim 1, which can be denoted as “ta-C” and typically has a sp³ fraction of above 60% can only be created if there is a high plasma density, in other words a high degree of ionization. The doped DLC according to the present invention can therefore not be obtained with standard unbalanced magnetron sputtering methods such as employed by Z. Wang et al. and K. Hou et al. For standard unbalanced magnetron sputtering, unlike for High Power Impulse Magnetron Sputtering (HIPIMS), the degree of ionization is too low.

The invention further provides a method of depositing a coating of non-hydrogenated DLC comprising at least one transition metal selected from groups 4d, 5d and 6d of the periodic table of elements. The method is a cathodic arc discharge deposition method. In the cathodic arc discharge, a direct current (DC) is superimposed with a pulsed current. The pulsed current has a pulse frequency in the range of 10 kHz to 100 kHz. A carbon target doped with the at least one transition metal is used as a target in the cathodic arc discharge. The target is connected directly to a cathode.

Each pulse of the pulsed current induces a rise of a voltage with a rate of more than 5 V/μs as measured on the cathode. Each pulse of the pulsed current has an active pulse width of less than 30 μs. In the method, the degree of ionization of the evaporated target material is close to 100%.

The dopant level of the target may be in the range of 0.5 at. % to 10.0 at. %, preferably in the range of 1.0 at. % to 8 at. %, more preferably in the range of 1.0 at. % to 6 at. % and more preferably in the range of 0.5 at. % to 2.5 at % of the at least one transition metal selected from groups 4d, 5d and 6d of the periodic table of elements. The at least one transition metal may be a transition metal as described above, such as tungsten (W). Using Electron Probe Microanalysis (EPMA), the present inventors found that about 60% of the transition metal being present in the target as dopant will be found in the doped DLC according to the present invention. For instance, 8 at. % W dopant in the target gave 5 at. % in the coating of the doped DLC according to the present invention, and 2 at. % W in the target gave 1.2 at. % in the coating.

Cathodic arc discharge deposition is a physical vapour deposition (PVD) technique in which an electric arc is generated between a target, which serves as a cathode or is connected to a cathode, and an anode. The electric arc evaporates material at a surface of the target in an area where the arc is present. The evaporated target material is deposited on a substrate so as to form a coating of the target material on the substrate. A. Anders, in the textbook “Cathodic ARCs”, Springer, 2008, ISBN 978-0-387-79107-4 provides a detailed introduction into cathodic arc discharge deposition.

The difference between cathodic arc and standard unbalanced magnetron sputtering is that the ionization of atoms to be deposited is much higher in the arc discharge than in the unbalanced magnetron discharge. The ionization of carbon (C) in a DC arc has been described by A. Andersin “Cathodic ARCs”, Springer (2008), ISBN 978-0-387-79107-4, paragraph 4.3 on pages 194-195, and is also for arc currents of 200 A already 100% singly ionized, and at higher currents even partially double ionized. The ionization of Carbon by Magnetron sputtering is much lower; typical values for C sputtering are in the range of 5%. ta-C with a higher fraction of sp³ has been described by Lifschitz et al. (Physical Review B, Vol. 41, No. 15, pp. 10468-10480) to be due to a sub-plantation process. Carbon atoms are arriving with an energy, sufficiently high to penetrate the surface of the growing film to a depth of typically 3 atom layers. At this depth the implanted atoms experience a high pressure and due to the high pressure sp³ bonds are formed. With standard (unbalanced) magnetron (non-HIPIMS) the average energy is too low due to the low ionization degree, to have sub-plantation for most C atoms.

In the present cathodic arc discharge deposition method, the cathodic arc discharge is generated or fed by supplying a direct current which is superimposed with a pulsed current.

By employing such a superposition of currents, the generation of macro-particles of the target material, i.e., macro-particles of carbon doped with the at least one transition metal, in the cathodic arc discharge process can be significantly reduced. Further, the occurrence of relatively deep craters with sharp edges on the target surface can be minimised. In particular, the pulses superimposed on the direct current cause a splitting of the electric arc into a plurality of arcs, jumping out of the crater of the first arc and rounding the edges by evaporation in this process, thus avoiding the formation of craters, especially deep craters, with sharp edges. Such sharp edges can be released from the target as macro-particles of the target material, thus affecting the quality of the coating to be formed. In the present method, the pulses of the pulsed current have a high rise rate. Hence, the splitting into a plurality of arcs occurs within a particularly short period of time.

Therefore, a high quality coating can be achieved. Moreover, since the formation of such macro-particles can be suppressed, there is no need to use a macro-particle filter in the deposition method, thus allowing for the method to be significantly simplified.

The target is connected directly to the cathode, i.e., without any intermediate layers or structures being present between the target and the cathode.

The pulsed current has a pulse frequency in the range of 10 kHz to 100 kHz, preferably in the range of 20 kHz to 90 kHz, more preferably in the range of 30 kHz to 80 kHz and even more preferably in the range of 40 kHz to 70 kHz. By choosing a pulse frequency in the range of 10 kHz to 100 kHz, the formation of macro-particles of the target material can be particularly efficiently and reliably suppressed.

For production applications, the cathodic arc discharge deposition process may be performed in a deposition chamber, in particular, a vacuum chamber, which is equipped with a plurality of cathodes. For example, if there is a bank of cathodes that are depositing simultaneously, the arc pulses can be synchronized with a delay set between the different arc sources, such that only one pulse at a time occurs, avoiding overload of a bias voltage power-supply.

In “Production of highly ionized species in high-current pulsed cathodic arcs” (R. Sanginés, A. M. Israel, I. S. Falconer, D. R. McKenzie, and M. M. M. Bilek, Applied Physics Letters 96, 221501 [2010]), it is shown, in FIG. 2 , that in pulsed arc deposition of Al with fairly long pulses of 600 μs and with a peak current of 800 A, doubly ionized Al is formed, but reaches its maximum already after 40 μs. It decreases then rapidly, with singly ionized Al left. The arc has continuously split at approximately every incremental 60 A.

The arcs have repelled each other. The fact that the double ionization has reduced considerably after 100 μs means that the plasma is similar to a DC arc after that period. In order to remain in the regime where higher ionization occurs and has not recombined with neutrals going back to standard DC arc conditions, the present method uses a high pulse voltage rise rate, so that all arcs are still close to each other, and the active pulse width of each pulse of the pulsed current is kept relatively short. The maximum time where the arc voltage still increases is less than 30 μs.

Each of the pulses of the pulsed current which is superimposed on the direct current induces a rise of a voltage, i.e., an arc discharge voltage, measured at the cathode with a rate of more than 5 V/μs. The voltage may be measured between the cathode and the anode. The anode may be at ground potential.

Using pulses inducing such a high voltage rise rate at the cathode, to which the target is connected directly, enables the generation of a plasma with a particularly high plasma intensity, plasma density and plasma temperature. By generating such a high density plasma, droplets of molten transition metal are formed at a surface of the target. Further, the high density plasma ensures that larger-size transition metal droplets released from the target surface are broken up into smaller-size droplets during their transfer from the target to the substrate to be coated, so as to reduce the droplet size. Hence, the quality of the coating can be further improved. In particular, the incorporation of such smaller-size droplets in the coating can considerably enhance the electrical conductivity of the coating, which is particularly beneficial, e.g., for a coating on a bipolar plate in a fuel cell or an electrolyzer. Further, the droplets can improve the friction reducing properties of the coating. Since the droplet size is reduced by the high density plasma during transfer of the droplets from the target to the substrate, the incorporation of large-size droplets in the coating can be avoided, thus obtaining a coating with a smooth and even surface structure.

Preferably, the rise rate of the voltage measured at the cathode which is induced by the pulses is more than 8 V/μs, more preferably more than 10 V/μs, even more preferably more than 12 V/μs, yet even more preferably more than 14 V/μs and still even more preferably more than 16 V/μs.

Each of the pulses of the pulsed current which is superimposed on the direct current has an active pulse width of less than 30 μs. The active pulse width of a pulse is defined as the time in which the arc current induced by the pulse is not yet decaying. The arc current induced by the pulse is measured at the cathode. The arc current induced by the pulse may start decaying when a pulse power supply supplying the pulse is switched off. By cutting off the pulses after such a relatively short time of less than 30 μs, it can be reliably avoided that the arc splitting enters a regime where the distances between arcs are bigger and the plasma density decreases to approximately the DC arc plasma density.

The pulses of the pulsed current may have an active pulse width of 1 μs or more and less than 30 μs, preferably in the range of 2 μs to 20 μs and more preferably in the range of 4 μs to 10 μs. Particularly preferably, the pulses of the pulsed current may have an active pulse width of 5 μs.

The direct current may be in the range of 50 A to 1000 A, preferably in the range of 100 A to 800 A, more preferably in the range of 200 A to 600 A and even more preferably in the range of 250 A to 500 A. Choosing a direct current in the range of 50 A to 1000 A can further enhance the formation of droplets of molten transition metal at the target surface.

The peak current of the direct current superimposed with the pulsed current may be higher than 200 A. The direct current superimposed with the pulsed current is measured at the cathode. By selecting a peak current of more than 200 A, the droplet size can be reduced by the high density plasma during transfer of the droplets from the target to the substrate in a particularly efficient manner.

The pulses of the pulsed current may have pulse separations in the range of 20 μs to 200 μs, preferably in the range of ps to 150 μs and more preferably in the range of 60 μs to 120 μs. Particularly preferably, the pulses of the pulsed current may have a pulse separation of 80 μs.

The method of the invention may be performed at a deposition temperature in the range of 50° C. to 180° C., preferably in the range of 70° C. to 150° C. The deposition temperature is measured at the substrate to be coated. Particularly preferably, the deposition temperature is kept between 70° C. and 150° C. By controlling the deposition temperature so as not to exceed 150° C., it can be particularly reliably ensured that the Young's modulus and the hardness of the coating do not decrease significantly.

A bias may be applied to the substrate in the cathodic arc discharge deposition process. The bias may be in the range of 10 V to 100 V, preferably in the range of 20 V to 80 V and more preferably in the range of 40 V to 60 V. Particularly preferably, the bias may be 50 V.

The background pressure in the cathodic arc discharge deposition process may be in the range of 1×10⁻⁵ mbar to 5×10⁻⁴ mbar, preferably in the range of 2×10⁻⁵ mbar to 1×10⁻⁴ mbar and more preferably in the range of 4×10⁻⁵ mbar to 6×10⁻⁵ mbar. Particularly preferably, the background pressure in the cathodic arc discharge deposition process may be 5×10⁻⁵ mbar.

The cathodic arc discharge deposition process may be performed in an atmosphere which contains an inert gas. More particularly, the cathodic arc discharge deposition process may be performed in an atmosphere which contains argon (Ar) or nitrogen (N₂). In particular, the cathodic arc discharge deposition process may be performed in a deposition chamber into which such a gas has been introduced. The background pressure of the inert gas, in particular, Ar, may be in the range of 1×10⁻⁴ mbar to 9×10⁻⁴ mbar, preferably in the range of 2×10⁻⁴ mbar to 8×10⁻⁴ mbar and more preferably in the range of 4×10⁻⁴ mbar to 6×10⁻⁴ mbar. Particularly preferably, the background pressure of the inert gas, in particular, Ar, may be 5×10⁻⁴ mbar. By performing the cathodic arc discharge deposition process in such an atmosphere, the ignition of the electric arc can be facilitated.

Prior to depositing the coating, the substrate to be coated may be cleaned, e.g., by ion etching, using Ar or metal ions.

The coating may be deposited directly on a surface of the substrate, i.e., without any intermediate layers being present between this surface and the coating. Alternatively, an initial adhesion layer may be provided on the surface of the substrate to be coated before depositing the coating thereon. The initial adhesion layer may be, for example, a metallic chromium (Cr) layer or a metallic titanium (Ti) layer. There is no limitation as to the method of applying the initial adhesion layer, and for example any CVD or PVD method, including sputtering, can be used.

The deposition parameters, such as the characteristics of the direct current and the pulsed current, the deposition temperature, the substrate bias, the gas atmosphere and the pressure, may be kept at least substantially constant during the cathodic arc discharge deposition process.

In some embodiments, the pulsed current may have a pulse frequency in the range of 20 kHz to 90 kHz. The dopant level of the target may be in the range of 0.5 at. % to 10.0 at. %. Each pulse of the pulsed current may induce a rise of a voltage with a rate of more than 8 V/μs as measured on the cathode. Each pulse of the pulsed current may have an active pulse width in the range of 2 μs to 20 μs. The peak current may be higher than 200 A. The pulses of the pulsed current may have pulse separations in the range of 20 μs to 200 μs. The method may be performed at a deposition temperature in the range of 50° C. to 180° C. A bias in the range of 10 V to 100 V may be applied to the substrate in the cathodic arc discharge deposition process. The background pressure in the cathodic arc discharge deposition process may be in the range of 1×10⁻⁵ mbar to 5×10⁻⁴ mbar.

The coating deposited by the method of the present invention is a coating of the doped DLC according to the present invention. The coating deposited by the method of the present invention may have the features, properties and characteristics described above.

It was found by the present inventors that the higher the content of transition metal, such as tungsten, in the target is, the more is relatively found in the coating of the doped

DLC according to the present invention as metal droplets. This is well understandable as on the target surface, under the influence caused by the arc, large particles of molten transition metal segregate alongside the arc track. When the arc hits such a transition metal particle, then droplets are emitted and broken up into smaller-size droplets due to the high density plasma caused by the high voltage rise rate of more than 10 V/μs, while the metal droplets travel from the target to the substrate to be coated, so as to reduce the droplet size. The percentage of transition metal, such as W, that segregates on the target surface as metallic droplets is more than linearly proportional with the dopant level of the target. This way, the relative proportions of the transition metal present in the form of carbide and the one present as a metal droplet in the doped DLC according to the present invention can be adjusted. FIG. 8 shows an example of a target exposed to the arc with 8 at. % of W. Small craters can be seen on the target surface. The white droplets are W.

The invention further provides a coating of the non-hydrogenated transition metal-doped DLC which is obtainable by the method of the present invention.

Embodiments

In the following, specific embodiments of the present invention will be summarized.

-   -   (1) A non-hydrogenated transition metal-doped diamond-like         carbon (DLC), wherein the non-hydrogenated metal-doped DLC         comprises at least one transition metal selected from groups 4d,         5d and 6d of the periodic table of elements and a part of the at         least one transition metal is present in the form of carbide of         the at least one transition metal in the non-hydrogenated DLC as         a matrix, and wherein the non-hydrogenated transition         metal-doped DLC has a hardness of 35 GPa, preferably of 40 GPa.         The hardness can be measured on a film of the non-hydrogenated         transition metal-doped DLC deposited on a polished substrate         with an indentation depth less than 10% of the thickness of the         film.     -   (2) The non-hydrogenated transition metal-doped DLC according to         item (1), wherein at least a part of the carbide of the at least         one transition metal is present as islands in the         non-hydrogenated DLC as a matrix.     -   (3) The non-hydrogenated transition metal-doped DLC according to         item (2), wherein the islands have a size of at most 2 nm.     -   (4) The non-hydrogenated transition metal-doped DLC according to         any one of items (1) to (3), wherein the non-hydrogenated DLC is         tetrahedral amorphous carbon, i.e. ta-C.     -   (5) The non-hydrogenated transition metal-doped DLC according to         any one of items (1) to (4), wherein another part of the at         least one transition metal is present in the form of the metal         as droplets of the transition metal.     -   (6) The non-hydrogenated transition metal-doped DLC according to         item (5), wherein the droplets of the transition metal have a         diameter of less than 1 μm, preferably of 0.1 to 100 nm,         preferably of 0.5 to 40 nm.

(7) The non-hydrogenated transition metal-doped DLC according to item (5) or (6), wherein a total of at least 85 at. %, preferably at least 90 at. %, of the at least one transition metal, is present in the matrix of the non-hydrogenated DLC in the form of carbide, preferably as islands of the carbide, and/or in the form of metal droplets.

-   -   (8) The non-hydrogenated transition metal-doped DLC according to         any one of items (1) to (7), which has a sp³ fraction of carbon         atoms of 60%, preferably of 70%, more preferably of 80%, and         most preferably of 85%.     -   (9) The non-hydrogenated transition metal-doped DLC according to         any one of items (1) to (8), wherein the transition metal is         selected from the group consisting of chromium, molybdenum and         tungsten, and is preferably tungsten.     -   (10) The non-hydrogenated transition metal-doped DLC according         to any one of items (1) to (9), wherein the content of the at         least one transition metal is in the range of 0.1 to 5 at. % in         terms of the non-hydrogenated transition metal-doped DLC,         preferably in the range of 0.2 to 2.5 at. %, more preferably in         the range of 0.3 to 2.0 at. %, most preferably in the range of         0.5 to 1.5 at. %.     -   (11) The non-hydrogenated transition metal-doped DLC according         to any one of items (1) to (10), the hardness of which is in the         range of 40 GPa to 60 GPa.     -   (12) A layer system comprising at least one layer of the         non-hydrogenated transition metal-doped DLC according to any one         of items (1) to (11) provided on a substrate.     -   (13) The layer system according to item (12), wherein the layer         is a homogeneous layer.     -   (14) The layer system according to item (12) or (13), wherein         the layer has a thickness in the range of 50 nm and 3 μm.     -   (15) The layer system according to any one of items (12) to         (14), which comprises a multilayer of         -   at least one layer of the non-hydrogenated transition             metal-doped DLC, in which the content of the at least one             transition metal is X at. % in terms of the layer, and         -   at least one layer of the non-hydrogenated transition             metal-doped DLC, in which the content of the at least one             transition metal is more than 0 to 0.8 times X at. % in             terms of the layer and/or at least one layer of ta-C.     -   (16) The layer system according to item (15), wherein the         multilayer has a thickness in the range of 0.1 μm to 30 μm,         preferably in the range of 0.2 μm to 10 μm.     -   (17) The layer system according to any one of items (12) to         (16), wherein an adhesion layer is provided directly on the         substrate, on top of which at least one layer of the         non-hydrogenated transition metal-doped DLC is formed.     -   (18) The layer system according to any one of items (12) to         (17), wherein the substrate is a metal substrate,     -   (19) The layer system according to any one of items (12) to         (18), wherein the metal substrate is a stainless steel         substrate, a titanium substrate or an aluminium substrate.     -   (20) The layer system according to item (19), wherein the metal         substrate is a stainless steel substrate.     -   (21) The layer system according to item (19), wherein the metal         substrate is a titanium substrate.     -   (22) A use of the non-hydrogenated transition metal-doped DLC         according to any one of items (1) to (11) for improving wear         resistance and/or reducing friction of a surface by applying a         coating of the non-hydrogenated transition metal-doped DLC on         the surface.     -   (23) The use according to item (22), wherein the         non-hydrogenated transition metal-doped DLC has a content of the         at least one transition metal in the range of 1 at. % to 5 at.         %.     -   (24) The use according to item (22) or (23), wherein the coating         has a thickness in the range of 0.5 μm to 3 μm.     -   (25) A method of depositing a coating of non-hydrogenated DLC         comprising at least one transition metal selected from groups         4d, 5d and 6d of the periodic table of elements, wherein the         method is a cathodic arc discharge deposition method, wherein in         the cathodic arc discharge, a direct current is superimposed         with a pulsed current, wherein the pulsed current has a pulse         frequency in the range of 10 kHz to 100 kHz, wherein a carbon         target doped with the at least one transition metal is used as a         target in the cathodic arc discharge, which target is connected         directly to a cathode, wherein each pulse of the pulsed current         induces a rise of a voltage with a rate of more than 5 V/μs as         measured on the cathode, and wherein each pulse of the pulsed         current has an active pulse width of less than 30 μs.     -   (26) The method according to item (25), wherein the peak current         is higher than 200 A.     -   (27) The method according to item (25) or (26), wherein the         coating is a coating of the non-hydrogenated transition         metal-doped DLC as defined in any one of items (1) to (11).     -   (28) The coating of the non-hydrogenated transition metal-doped         DLC as defined in any one of items (1) to (11), which is         obtainable by a method as defined in item (25) or (26).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a cathodic arc discharge deposition apparatus for performing a deposition method according to an embodiment of the present invention.

FIG. 2 shows Raman spectra of specific examples of coatings of the doped DLC according to the present invention. The samples had different dopant levels, i.e. contents of transition metal, namely, W, as follows: B1: 0 at. % (for reference); B2: 0.3 at. %; B3: 0.6 at. %; B4: 1.4 at. %; B5: 1.4 at. %. The Raman excitation wavelength was 532 nm with a 50 cm⁻¹ edge filter.

FIG. 3 is a Scanning Electron Microscope (SEM) image of 80 nm W doped ta-C with a W concentration of 1.4 at. % according to a concrete example of the doped DLC according to the present invention. Visible is the wafer window with Si₃N₄ film, on top of which the doped ta-C is deposited. The dark balls are W.

FIG. 4 is a high resolution transmission electron microscope (HR TEM) picture in HAADF-STEM mode of ta-C doped with 1.4 at. % W according to an example of the doped DLC according to the present invention. Due to focusing, only a slab of 20 nm is observed as a projection.

FIG. 5 is the HR-TEM bright field of 80 nm 5 at. % W doped ta-C on top of 20 nm Si₃N₄ according to a specific example of the doped DLC according to the present invention. The dark balls are W droplets.

FIG. 6 is a TEM micrograph of 80 nm 5 at. % W doped ta-C in dark field mode according to a specific example of the doped DLC according to the present invention. Bright spots are pure W droplets embedded in a ta-C matrix.

FIG. 7 is a HAADF-STEM photograph of 5 at. % W doped ta-C according to a specific example of the doped DLC according to the present invention. W shows up white as the micrograph is taken in dark field mode.

FIG. 8 is a SEM image of a graphite target comprising 8 at. % W after use in the cathodic arc discharge deposition method according to the present invention. The white droplets are W.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Deposition Method

In the following, a method of depositing a coating of non-hydrogenated DLC comprising at least one transition metal selected from groups 4d, 5d and 6d of the periodic table of elements according to an embodiment of the present invention will be described with reference to FIG. 1 .

In the present embodiment, the method is performed using a cathodic arc discharge deposition apparatus 2 as shown in FIG. 1 . The apparatus 2 comprises a deposition chamber 11, an anode 4, a target 6, a cathode 7, and a current source 8 electrically connected with the anode 4 and the cathode 7. The cathode 7 may be made of a metal, such as copper (Cu). For example, the cathode 7 may be a metal plate, e.g., a Cu plate. The cathode 7 may be cooled, for example, water-cooled. The anode 4 is integral with a wall of the deposition chamber 11. The anode 4 may form part of the wall of the deposition chamber 11. The anode 4 may be made of the same material as the wall of the deposition chamber 11, e.g., a metal, such as steel. The anode 4 has a substantially annular shape which is shown in cross-section in FIG. 1 . For example, the anode 4 may be a substantially annular metal plate, e.g., a substantially annular steel plate. In particular, the anode 4 may be a substantially annular flat plate, such as a substantially annular flat metal plate. The anode 4 and the cathode 7 are arranged in a plane which is substantially perpendicular to a direction from the cathode 7 towards the target 6 (see FIG. 1 ). The cathode 7 is arranged within a central opening of the substantially annular anode 4.

The target 6 is connected directly to the cathode 7, i.e., without any intermediate layers or structures being present between the target 6 and the cathode 7. In particular, a surface, e.g., a backside surface, of the target 6 may be in contact, e.g., full contact, with the cathode 7, e.g., the body of the cathode 7. The target 6 may be mounted onto the cathode 7, for example, by a bolt connection (not shown). The target 6 is electrically connected directly to the cathode 7, e.g., by a direct contact between the backside surface of the target 6 and the cathode body.

The current source 8 is configured to supply a direct current which is superimposed with a pulsed current, so as to generate an electric arc between the target 6 and the anode 4. The electric arc evaporates material at a surface 10 of the target 6 in an area where the arc is present. The evaporated target material 12 (see FIG. 1 ) is transferred from the target 6 to a substrate 14 and deposited on the substrate 14 so as to form a coating of the target material thereon.

The anode 4, the target 6, the cathode 7 and the substrate 14 are arranged within a space formed inside the deposition chamber 11. The deposition chamber 11 may be a vacuum chamber, e.g., an ultrahigh vacuum (UHV) chamber. The anode 4 is integral with and connected, i.e., electrically connected, with the wall of the deposition chamber 11. The current source 8 is electrically connected with the anode 4 via the wall of the deposition chamber 11 (see FIG. 1 ). In particular, the wall of the deposition chamber 11 may be made of a conductive material, such as a metal, thus establishing an electrical connection between current source 8 and anode 4. In other embodiments, a plurality of cathodes 7 may be arranged in the deposition chamber 11.

The background pressure in the deposition chamber 11 may be 5×10⁻⁵ mbar. The cathodic arc discharge deposition process of the present embodiment may be performed in an atmosphere in the deposition chamber 11 which contains an inert gas, in particular, Ar. For example, the pressure of Ar in the deposition chamber 11 may be 5×10⁻⁴ mbar.

In the present embodiment, the target 6 is a carbon target doped with tungsten (W). Hence, a coating of non-hydrogenated DLC comprising W is formed on the substrate 14 by the deposition process. The dopant level of the target 6 is in the range of 0.5 at % to 8.0 at % W.

The superimposed currents supplied by the current source 8 have the following properties. A direct current of 50 A is superimposed with a pulsed current having a pulse frequency in the range of 10 kHz to 100 kHz. The pulses of the pulsed current have an active pulse width of 5 μs and a pulse separation of 80 μs. The peak current of the direct current superimposed with the pulsed current is higher than 200 A. The direct current and the pulsed current are measured at the cathode 7.

Each of the pulses of the pulsed current induces a rise of a voltage, i.e., an arc discharge voltage, measured at the cathode 7 with a rate of more than 5 V/μs. The voltage is measured between the cathode 7 and the anode 4. The anode 4 is at ground potential.

By employing such a superposition of currents, the generation of macro-particles of the target material in the cathodic arc discharge process can be significantly reduced. Further, the occurrence of craters on the target surface can be minimised. Moreover, it can be ensured that small-size droplets of molten W are incorporated in the coating, thus enhancing the electrical conductivity and the friction reducing properties of the coating. Hence, a particularly high quality coating can be provided on the substrate 14.

The pulses superimposed on the direct current cause a splitting of the electric arc into a plurality of arcs, as has been detailed above. In the present embodiment, the current, i.e., the arc current, of each single arc obtained by this splitting may be, for example, approximately 60 A. At the peak current, the total number of arcs may be, e.g., five or six. If the arc current is maintained at a high level, the arcs obtained by the pulse-induced arc splitting repel each other and the plasma characteristics become similar to those of a DC arc. Therefore, the active pulse width is kept short, i.e., below 30 μs. In the present embodiment, the active pulse width is 5 μs, as has been detailed above.

In the present embodiment, the cathodic arc discharge deposition process is performed at a deposition temperature in the range of 70° C. to 150° C. The deposition temperature is measured at the substrate 14. Further, a bias of 50 V is applied to the substrate 14.

The deposition parameters, such as the characteristics of the direct current and the pulsed current, the deposition temperature, the substrate bias, the gas atmosphere and the pressure, are kept substantially constant during the cathodic arc discharge deposition process of the present embodiment.

Prior to depositing the coating, the substrate 14 may be cleaned, e.g., by Ar etching. The coating may be deposited directly on a surface of the substrate 14. Alternatively, an initial adhesion layer may be provided on the surface of the substrate 14 before depositing the coating thereon. The initial adhesion layer may be, for example, a metallic chromium (Cr) layer or a metallic titanium (Ti) layer. From initial adhesion layer to doped Carbon layer, one could have an abrupt transition or a ramp down of the metal deposition rate and a ramp up of the doped Carbon deposition rate.

Examples

The present invention will be further illustrated by way of examples, which of course must not be construed in a limiting sense.

The coatings are deposited by running a cathodic arc discharge on a carbon target doped with W. The arc discharge was a DC arc with a pulse superimposed. Samples were prepared with a DC arc current of 50 A, superimposed with a pulse with a width of 5 μs, with pulse separation of 80 μs, and an arc peak current higher than 200 A. Dopant levels of W in between and 5 at. % W have been applied. The deposition temperature is kept in between 70° C. and 150° C. The bias energy voltage applied to the substrate was 50 V. The background pressure was typically 5×10⁻⁵ mbar. To have proper arc ignition a small quantity of Ar is allowed in the chamber, coming typically to an Ar pressure of 5×10⁻⁴ mbar.

The products, i.e. substrates, to be coated are cleaned by Ar etching, i.e. argon ions bombardment, prior to the deposition. Addition of an initial adhesion layer like metallic Cr or metallic Ti has been applied for a number of samples, but also coatings without the adhesion layer have been produced.

The ta-C coatings were deposited without changing any of the parameters during the deposition step. That means that the conditions, like pressure, gas atmosphere, bias Voltage and substrate temperature were kept constant during the deposition process.

Analysis Composition Analysis

The composition analysis of the coatings has been done by Electron Probe Micro Analysis EPMA with an acceleration Voltage of 5 keV, a current of 200 nA, and 10 test probes per sample. The compositions studied in detail are shown in Table 1.

TABLE 1 Overview of coatings studied Indentation Sample hardness H_(IT) in number Composition GPa Raman WC peak B1 ta-C 54 none B2 ta-C + 0.3 52.4 none at % W B3 ta-C + 0.6 43.4 slightly at % W B4, B5 ta-C + 1.4 47.0 clearly visible at % W

Hardness

The hardness is an indentation hardness H_(IT) and has been measured by nano-indentation on a flat polished hardened substrate with a micro hardness tester (Fischerscope H100) according to ISO 14577 (namely, the English versions of ISO 14577-1:2015 of 15 Jul. 2015, ISO 14577-2:2015 of 15 Jul. 2015, and ISO 14577-4:2016 of Nov. 1, 2016) using a diamond indenter. The diamond indenter used has a Vickers geometry. That is, the indenter is a diamond with a square based pyramid shape with planes at 22° relative to the horizontal plane or in other words it is shaped as an orthogonal pyramid with a square base and with an angle of 68° between the axis of the diamond pyramid and one of the faces (Vickers pyramid). The hardness is expressed in GPa. The roughness Ra of the substrate was less than 0.06 μm. Specifically, the substrate (test plate) had a surface roughness Ra of 0.01 μm and Rz of 0.25 μm. The hardness of the flat polished hardened substrate used was 83.6 HRa (Rockwell hardness A, HRA), 62.1 HRc (Rockwell hardness C, HRC) and 747 HV10 (Vickers hardness at a load of 10 kgf). The size of the test plate, i.e. the flat polished hardened substrate, was 15×6 mm. The load of the indenter and the film thickness of the coating deposited on the test plate were selected such, that the indentation depth was less than 10% of the coating thickness. Ten points per sample (uniformly distributed over the surface of the coating to be measured) were taken for the measurement and the average of the ten hardness indentation values H_(IT) thus measured is used (unit is GPa). The relation between indentation hardness H_(IT) in GPa and Vickers hardness (Hv) is:

Hv=94.53 H_(IT).

Raman Test

Raman tests were done on samples before and after the corrosion steps and with different dopant contents. The Raman excitation wavelength was 532 nm. The position of the G peak was at 1605 cm⁻¹, which for that excitation wavelength points to a sp³ fraction over 60%. In between 80-150 cm⁻¹ there is a peak becoming visible, representing WC bonds. The pure ta-C sample and the one with 0.3 at % W does not show this peak. For 0.6 at % W and 1.4 at % W it is visible and is stronger for higher W content. Raman does not provide information whether the WC bonds are pointing to WC crystallites, or to bonds between individual W atoms with C. In tests of the coatings after a corrosion step, no substantial difference in Raman spectra pre- and post-oxidation could be seen.

TEM Studies of Samples

TEM studies were done with a beam Voltage of 200 kV with bright field TEM (BFTEM) and with high angle annular dark field with spot for scanning TEM (HAADF-STEM). The samples consisted of silicon (Si) wafers with a 20 nm thick Si₃N₄ layer on top, where at 1×1 mm² the Si was etched away. In this way windows are present. On top of the Si₃N₄ foil 80 nm ta-C was deposited doped with respectively 0.3, 0.6 and 1.4 10 at % W. The Si₃N₄ foil was mostly broken under influence of the compressive stress of the doped ta-C film, but we could find undisturbed foil in the corners of the windows. The observation was made from the side of the coating of the doped DLC according to the present invention deposited on the Si₃N₄ foil.

The combination of BFTEM and HAADF-STEM imaging allowed for discrimination between W droplets and carbon clusters.

W droplets were identified in the 80 nm film. FIG. 3 shows a coating doped with 1.4 at % W observed with BFTEM. The small black spots are W droplets as confirmed by EDX analysis. In FIG. 3 the W droplet size ranges from 2 to 40 nm. In other samples droplets up to 100 nm were observed.

In FIG. 4 a HAADF-STEM image of the same sample doped with 1.4 at. % W is shown. Sub-nm clusters of W were recognizable (in HAADF-STEM W shows up as white). It should be realized that due to the focus of the HRTEM not the total film thickness of 80 nm, but only a slot with a thickness of approximately 20 nm was observed. Although an accurate measurement of such particles in a thick coating is not possible, a rough measurement yielded a cluster size of −0.5 nm. W has a bcc structure with lattice spacing 0.34 nm. WC has normally a hexagonal distribution with lattice parameters 0.29 and 0.28 nm. So, the clusters are made up of a pretty limited number of W atoms, in the range of up to 10.

A semi-quantitative analysis was made of the amount of W visible in metallic droplets and the amount visible in WC islands, assuming that islands and droplets are spherical. For a coating of the doped DLC according to the present invention doped with 0.6 at % W, it was found that 50% of W could be attributed to WC islands and 50% of W in metallic droplets. For a film doped with 1.4 at % W approximately 30% of W could be attributed to WC islands and 70% to metallic W droplets. It is in line with the expectation that the amount of droplets increases more than linearly with the percentage of W in the target.

With the very clear presence of WC in the Raman spectra also for only 1.4 at % W, it is clear that the smaller crystallites are WC. The quantitative analysis did indicate that there is hardly any “free” W.

For studying the tribological properties of the coating of the doped DLC according to the present invention, coatings were deposited as described above, this time realizing a content of W as an example of the transition metal of 5 at. %, in terms of the W-doped DLC.

Via HR-TEM and TEM, the presence of droplets of W could be confirmed for the sample doped with 5 at. % W. Typical TEM micrographs of the sample are shown in FIGS. 5 and 6 . In the bright field HR-TEM image of FIG. 5 , the pure W particles appear black, and in the TEM micrograph of FIG. 5 taken in dark field mode, the pure W particles appear as bright spots. The maximum particle size observed for this sample was below 250 nm. The particles were analysed by EDX showing that they are W droplets.

In FIG. 7 a HAADF-STEM image of the sample doped with 5 at. % W is shown. Sub-nm clusters of W were recognizable (in HAADF-STEM W shows up as white). It should be noted that due to the focus of the HRTEM not the total film thickness of 80 nm, but only a slot with a thickness of approximately 20 nm could be observed. Moreover, the presence of WC in the Raman spectra was confirmed as detailed above. It is thus clear that the sub-nm clusters of W are WC.

It has thus been shown that the doped DLC according to the present invention has a much higher hardness than the transition metal-doped hydrogenated DLC used in the prior art, such as by A. Abou Gharam et al. for tribological applications. Moreover, the droplets of the transition metal are shown to have a small size and are incorporated in the non-hydrogenated DLC as a matrix, so that a coating with a smooth and even surface structure can be obtained, which additionally contains droplets of the transition metal having a lubricating effect. 

1. A non-hydrogenated transition metal-doped diamond-like carbon (DLC), wherein the non-hydrogenated DLC comprises at least one transition metal selected from groups 4d, 5d and 6d of the periodic table of elements and a part of the at least one transition metal is present in the form of carbide of the at least one transition metal in the non-hydrogenated DLC as a matrix, characterized in that the non-hydrogenated transition metal-doped DLC has a hardness of 35 GPa, preferably of 40 GPa wherein the hardness is measured on a film of the non-hydrogenated transition metal-doped DLC deposited on a polished hardened substrate with an indentation depth less than 10% of the thickness of the film, and another part of the at least one transition metal is present in the form of the metal as droplets of the transition metal.
 2. The non-hydrogenated transition metal-doped DLC according to claim 1, wherein a part of the carbide of the at least one transition metal is present as islands in the non-hydrogenated DLC as a matrix, which islands preferably have a size of at most 2 nm.
 3. The non-hydrogenated transition metal-doped DLC according to claim 1 or 2, wherein the metal droplets of the transition metal have a diameter of less than 1 μm, preferably of 0.1 to 100 nm and more preferably of 0.5 to 40 nm.
 4. The non-hydrogenated transition metal-doped DLC according to any one of claims 1 to 3, wherein the content of the at least one transition metal is in the range of 0.1 to 5 at. %.
 5. The non-hydrogenated transition metal-doped DLC according to any one of claims 1 to 4, the hardness of which is in the range of 40 GPa to 60 GPa.
 6. The non-hydrogenated transition metal-doped DLC according to any one of claims 1 to 5, which has a sp³ fraction of carbon atoms of 60%, preferably of 80%.
 7. The non-hydrogenated transition metal-doped DLC according to any one of claims 1 to 6, which has a content of the at least one transition metal in the range of 1 at. % to 5 at. %.
 8. A layer system comprising at least one layer of the non-hydrogenated transition metal-doped DLC according to any one of claims 1 to 7 provided on a substrate.
 9. The layer system according to claim 8, wherein the layer of the non-hydrogenated transition metal-doped DLC has a thickness in the range of 50 nm to 3 μm.
 10. A use of the non-hydrogenated transition metal-doped DLC according to any one of claims 1 to 7 for improving wear resistance and/or reducing friction of a surface by applying a coating of the non-hydrogenated transition metal-doped DLC on the surface.
 11. The use according to claim 10, wherein the coating has a thickness in the range of 0.5 μm to 3 μm.
 12. A method of depositing a coating of the non-hydrogenated transition metal-doped DLC as defined in any one of claims 1 to 7, wherein the method is a cathodic arc discharge deposition method, wherein in the cathodic arc discharge, a direct current is superimposed with a pulsed current, wherein the pulsed current has a pulse frequency in the range of 10 kHz to 100 kHz, wherein a carbon target doped with the at least one transition metal is used as a target in the cathodic arc discharge, which target is connected directly to a cathode, wherein each pulse of the pulsed current induces a rise of a voltage with a rate of more than 5 V/μs as measured on the cathode, wherein each pulse of the pulsed current has an active pulse width of less than 30 μs, and wherein preferably the peak current is higher than 200 A .
 13. The coating of the non-hydrogenated transition metal-doped DLC as defined in any one of claims 1 to 7, which is obtainable by the method as defined in claim
 12. 